Poly(ethylene terephthalate) (PET) films are widely used for a variety of wrapping, packaging, and lamination applications. Sometimes, PET film is used in shrink wrap applications in which the film is applied to an object and heated so that the film shrinks around the object. In other applications, such as flexible electronic circuit boards, heat resistant packaging, and cook-in bags, biaxially-oriented and heat-set PET film having good dimensional stability and shrink resistance at elevated temperatures is used. However, biaxially-oriented PET films are not useful at temperatures exceeding 250° C. because their inherent melting temperature (Tm) is 250° C.
Certain applications, such as certain flexible circuit boards, require films that are heat stable (i.e., possessing good dimensional stability) at 260° C. Specifically, the films must not blister or wrinkle when immersed in a solder bath preheated to 260° C. More specifically, these films must undergo 3% or less shrinkage when immersed for 10 seconds in a solder bath preheated to 260° C. Films that meet this requirement are combined with adhesive and circuitry into a flexible laminate that can then be subjected to a wave or dip solder at 260° C. Any blistering or wrinkling of the base film of this laminate during soldering at 260° C. may affect circuit performance in the final application.
Flexible circuit boards commonly use base and cover films in thicknesses of 70-150 microns (3-5 mils). In order to obtain the desired final film properties, these films are obtained by stretching an essentially amorphous cast film at ratios from about 2.5×2.5 to 3.5×3.5 (or more specifically, about 2.5×2.5 to 3×3). This would require the essentially amorphous films to be cast to between about 450-1800 microns (18-70 mils), or more specifically between 450-1400 microns (18-55 mils). The film must first be extruded essentially amorphous prior to stretching in order to prevent breakage during the stretch process. Unfortunately, pure poly(cyclohexylene dimethylene terephthalate) (PCT) crystallizes much more quickly than PET. As such, it is difficult to obtain an essentially amorphous cast or polished film at thicknesses of greater than 400 microns (16 mils). It should be noted that it is not necessary for the edges of the film to be fully amorphous, but only the center of the cast film.
Superior hydrolytic stability is another desirable property for films used in flexible circuit boards, particularly for automotive applications, and cook-in bags. Base films with superior hydrolytic stability produce circuit boards and bags that will maintain their structural integrity. In addition, it is desirable for the substrate and cover films used in flexible circuit boards to be insulative materials, to prevent charge from bridging across the circuitry. The insulative capability of a film used in these applications is measured by the dielectric constant. It is desirable for a film used in flexible circuit board applications to have as low a dielectric constant as possible.
Thus, it is an object of the present invention to provide a composition that can be cast into an essentially amorphous film at thicknesses about 450 microns (18 mils) or greater. It is also an object of the invention to provide a film that is stable at 260° C., that has superior hydrolytic stability, and that has a low dielectric constant. These and other objects will be apparent from the following description and claims of the invention.